The present invention relates to a sign readable at speed and is concerned with such a sign which, for example, is suitable for location by the side of a railway track for presenting information to a viewer of the sign travelling along the track.
U.S. Pat. Specification No. 3,568,346 discloses an optical sign of this type in which the functional assembly of the sign comprises an array of hexagonal or rectangular spherical biconvex lenses of positive power, and a corresponding tessellated array of identical optical objects, each object being disposed in the focal plane of the corresponding lens and being a representation of the information which the sign is to present. However the use of spherical biconvex lenses is disadvantageous due to spheric aberrations.
In order to reduce spheric aberrations British Patent Specification No. 2,149,527 provides an optical sign comprising hexagonal or rectangular lenses which are aspheric and optionally may be of the equivalent fresnel design. The optical object corresponding to each lens is disposed on a hexagonal sheet that conforms to the contour of the focal surface of the lens and is concave with respect to the front of the sign.
However providing the optical object on a surface which conforms to the focal surface of the aspheric lens is difficult.